Tin Boxes

Actually you don't need to put bracing or other support under the soundboard of a tin box guitar. It's true that tin can be flimsy in certain ways but it is very strong in others. The key is how you use it.

Think for a moment about instruments with membrane type sounding surfaces (eg. banjos and a whole range of African instruments). They don't have bracing under their bridges and the stuff they're made from is much more flexy than tin. Think also about resonator guitars. The cones in proper reso guitars are made of very thin metal - although they derive some strength from their dished or conical shape they still have a bit of give in them. You need to tension strings evenly and be prepared to do a bit of gradual fine adjustment until the instrument settles fully into tune. That doesn't mean they can't be great guitars.

Well, there can be a lot of parallels between the above and tin guitars.

The basic engineering is quite simple. Tension in the strings produces a downward point load through the bridge. This is balanced by tension in the soundboard (either the tin lid or base). Tension in the soundboard pulls on the sides of the tin. The problem is really with the sides of the tin, rather than the top or base. Tin is strong in tension but weak in terms of bending resistance. Unless you do something to support or stiffen the sides they will tend to bend inwards and you'll have a flexing, hard-to-tune, hard-to-play instrument.

There are at least two things you can do:

(1) Brace the tin internally (so as to provide support at the places indicated by the arrows above - not under the bridge)

(2) Use tins with curved or shaped sides - because giving certain types of shape to a thin sheet can imbue it with much greater resistance to bending. (modern cars are a great example - the sheet metal they're made out of will flop around like so much tin foil until it's pressed into shape).

I've tended to combine these techniques. The box I used for the tin guitar pictured in my earlier post had a "wavy" outline to its edges, which seems to make them quite stiff. There is also some bracing support provided by the thru' neck. But it is just the neck - no additional wood bracing was needed for strength purposes.

Other types of tins that will benefit from the strength inherent in their shape are the many circular tins available, and also those with a sort of "distorted circular" shape like the one in the picture from Dr X.

To sum up, what I'm saying is that you can leave the soundboard free to vibrate as long as you ensure that the rest of the guitar is sufficiently stiff - and you might be able to do that by just using a conveniently shaped tin. The soundboards of my guitars are completely free and unsupported (the thru' necks being carved away so they're not in contact apart from at the every edges). I've found that works very well.

Replies

David Bowes March 23, 2013 at 5:31am

I am using a section of threaded rod about 8mm as a bridge on a tin body, my question is should I have a flat bottom on it?

I read some where you should.

David
- Dr X > David Bowes March 23, 2013 at 5:58am
  
  Metal on metal will be.quite skiddy so a flat to stop it rolling would make sense, if you arent having any bridge support, id be tempted to make it a little longer than needed and round off any sharp edges.. so as not to punch/dent the skin when under string pressure,
  
  Having said that i find a wooden bridge adds warmth to the tone.
- David Bowes > Dr X March 23, 2013 at 6:28am
  
  Dr X
  
  Thanks, I was considering a wooden bridge, but was unsure it would work with a tin lid. I think I will give a wooden bridge a ago, as it will also be easier get a flat bottom and to adjust the height.
  
  Thanks
- Dr X > David Bowes March 23, 2013 at 6:48am
  
  Mahogany, with a piece of fret wire lightly grooved for strings is my default.
  Id love to see some pics =)
- David Bowes > Dr X March 23, 2013 at 7:05am
  
  Nice idea,I was just pondering how to do it.
  
  David
- The Phrygian Kid > David Bowes March 23, 2013 at 7:51am
  
  I really like the sound of metal bridges on tin gitts. Try a bent spoon. See my pics for many examples
Mark March 21, 2013 at 8:55am

Actually you don't need to put bracing or other support under the soundboard of a tin box guitar. It's true that tin can be flimsy in certain ways but it is very strong in others. The key is how you use it.

Think for a moment about instruments with membrane type sounding surfaces (eg. banjos and a whole range of African instruments). They don't have bracing under their bridges and the stuff they're made from is much more flexy than tin. Think also about resonator guitars. The cones in proper reso guitars are made of very thin metal - although they derive some strength from their dished or conical shape they still have a bit of give in them. You need to tension strings evenly and be prepared to do a bit of gradual fine adjustment until the instrument settles fully into tune. That doesn't mean they can't be great guitars.

Well, there can be a lot of parallels between the above and tin guitars.

The basic engineering is quite simple. Tension in the strings produces a downward point load through the bridge. This is balanced by tension in the soundboard (either the tin lid or base). Tension in the soundboard pulls on the sides of the tin. The problem is really with the sides of the tin, rather than the top or base. Tin is strong in tension but weak in terms of bending resistance. Unless you do something to support or stiffen the sides they will tend to bend inwards and you'll have a flexing, hard-to-tune, hard-to-play instrument.

There are at least two things you can do:

(1) Brace the tin internally (so as to provide support at the places indicated by the arrows above - not under the bridge)

(2) Use tins with curved or shaped sides - because giving certain types of shape to a thin sheet can imbue it with much greater resistance to bending. (modern cars are a great example - the sheet metal they're made out of will flop around like so much tin foil until it's pressed into shape).

I've tended to combine these techniques. The box I used for the tin guitar pictured in my earlier post had a "wavy" outline to its edges, which seems to make them quite stiff. There is also some bracing support provided by the thru' neck. But it is just the neck - no additional wood bracing was needed for strength purposes.

Other types of tins that will benefit from the strength inherent in their shape are the many circular tins available, and also those with a sort of "distorted circular" shape like the one in the picture from Dr X.

To sum up, what I'm saying is that you can leave the soundboard free to vibrate as long as you ensure that the rest of the guitar is sufficiently stiff - and you might be able to do that by just using a conveniently shaped tin. The soundboards of my guitars are completely free and unsupported (the thru' necks being carved away so they're not in contact apart from at the every edges). I've found that works very well.
- The Phrygian Kid > Mark March 23, 2013 at 7:42am
  
  This is exactly right. I've never braced the lid and I've done over a hundred of em
- Dr X > Mark March 22, 2013 at 10:19am
  
  WOW, i feel a bit dim now mark =] .. makes perfect sense, thank you for such a comprehensive answer
  
  i have three builds at a stage where i can impliment these ideas ... watch this space.
Michael Fred Johnson March 18, 2013 at 7:16am

of 3

This reply was deleted.

The Brits are coming

Replies